The Bottled Milk is part of the Farming Bundle in Forager, and you will need a total of 50 Bottled Milk in order to complete this part of the bundle. But how do you even get milk in Forager? The answer is pretty simple, and all you need to do is to use a Bottle while having your mouse hovering over a cow.
Clicking on the cow with a Bottle selected will consume the Bottle and give you a Bottled Milk instead. You can get as many Bottled Milk from a single cow as you want, and there is no need to wait for a recharge; just keep clicking it until you got enough. The big problem is getting enough Bottles, and it can be pretty difficult to get over 50 Bottles in the early game.
Besides being used for the bundle, the Bottled Milk will restore your energy if you drink it, which can be a nice option if you are out of other food.
There is no way to get your Bottle back if you accidentally click on a cow while having it in your active slot, so be careful.